Dr. Manette is the attending physician for the Evremonde brother who has mortally wounded the brother of a youthful Madame DeFarge. As the son of the Evremonde twin, Charles Darnay is called to retribution because Madame Defarge wishes to exterminate the Evremondes just as they destroyed her family. This lust for vengeance in Mme. DeFarge has stored up its potency through long years of malevolent brooding, just as the revolution in France has been brooding for years. Thus, the French Revolution and its guillotine are perfect vehicles for her private war against the Evremonde family.

mwestwood has answered this question perfectly, and I have very little to add except to say that this letter that Dr. Manette wrote so long ago is a key plot device of the novel that Dickens only reveals at a crucial point in the plot. We as readers have many questions as we read the novel, and this letter answers the majority of them. Why was Dr. Manette imprisoned? Why is Mme Defarge so focussed on revenge, especially when it comes to the Evremonde family? In a brilliant twist, and highly ironic when we remember that this letter is revealed when Dr. Manette himself is trying to save his son in law, the letter is read out and we find answers to these troubling questions.

The letter is also the proof the court needs to keep Charles Darnay in prison and eventually to be killed. When Dr. Manette, certainly not an aristocrat then or now, curses the entire Evremonde family, he is in effect condemning Charles to die--despite the fact that Charles was a mere boy, grew into a man with an entirely different set of principles, tried to help those whom he knew his uncle had wronged, and disowned his entire family estate. Without it, Therese would have had a difficult time just telling her story and claiming the right to kill Charles, the last living Evremonde. It is a device Dickens uses to create an element of surprise, as mentioned, and it is a tool to give the audience a satisfying justification for Madame Defarge's mad lust for revenge; it's also the death sentence for a good man who got caught up in the mad hunt to kill aristocrats.

I think that the letter is one of Dickens's most interesting and creative storylines. It makes this book a real mystery. Just when you think you have things figured out, the mysterious letter resurfaces. The letter so beautifully ties all of the story lines together. It is one of the reasons why I think this is one of Dickens's most taut and brilliant works.

Am I the only one disturbed by Dicken's implementation of the letter as plot device. Several problems with a few quotes to start:

1) It was written "in the last month of the tenth year of my captivity," yet "my memory is exact."

2) "... formed by the rusty iron point with which I write with difficulty in scrapings of soot and charcoal from the chimney, mixed with blood...."

3) "These scraps of paper fail me. One was taken from me, with a warning, yesterday. I must finish my record to-day."

Dr. Manette, though learned, and clearly disturbed in his 15th year has perfect recall in his 10th year of captivity? He relays over 5000 words (today, ~300 typed, ~400 written words per page), on what would be 10-20 pages given ideal conditions, but with rust and charcoal, which would have doubled or trebled the number of full pages (though "scraps), of which he had some taken.

His letter would not have wasted precious space indicating that it was difficult to write, or many other unnecessary comments, nor with the full prose of Dicken's ornate style.

Note: It is not the letter itself as plot device, but the inconsistency with Manette's probably mental state and memory (should have been written years earlier), the improbability of having a ream of paper at his disposal (letter was far too long), and all the loose ends conveniently tied together in one device, forcing the length, which I found less than satisfying and inconsistent with this masterpiece.